Looking at the api documentation found HERE and HERE, you are calling the /gridpoints/{wfo}/{x},{y}/forecast/hourly call which will return the hourly weather forecast for the specified weather office {wfo} at the specified x-y coordinates. You can find a list of the weather offices HERE. Finding the X-Y coordinates for the weather offices may be a bit more tedious to find on the web.

If you happen have access to the GPS coordinates that you are working with you can use the /points/{x},{y} API call to get the information on the closest weather office to then pass to the /gridpoints/{wfo}/{x},{y}/forecast/hourly API call.

The flow of your application can look something like this:

Step 1: Get your map Geo coordinates. In my case, I am at 35,-106

Step 2: Make a call to the weather.gov API: https://api.weather.gov/points/35,-106. You will be presented with some JSON data. Look for the cwa key in the properties object. That will be the forecast office to pass into the next api call. In my case, the key is ABQ. You also need to find the gridX and gridY keys in the properties. These are the XY coordinates that you will use for the {X},{Y} parameters in the API call. In my case X = 121 and Y = 112.

Step 3: Make the final call to the weather.gov API: https://api.weather.gov/gridpoints/ABQ/121,112/forecast/hourly

It seems your site changed policy. It requires User-Agent header be set. All you need is to set it to some value.

var url = "http://forecast.weather.gov/MapClick.php?lat=42&lon=-75&FcstType=dwml";
HttpClient client = new HttpClient();
client.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", "Stackoverflow/1.0");
var xml = await client.GetStringAsync(url);